Silicon photomultipliers (SiPMs), which are also commonly referred to as MicroPixel Photon Counters (MPPC) or MicroPixel Avalanche Photodiodes (MAPD) have become popular for use as photosensors in positron emission tomography (PET) scanners. SiPMs are tiled arrays of up to tens of thousands of avalanche photodiodes of typical size between 10 to 100 microns, connected in parallel on a common silicon substrate and working on common load. The output of an SiPM device is typically connected to a buffer amplifier, which can be implemented as a transimpedance amplifier. In PET detectors, several SiPM devices are grouped together and are optically coupled to scintillator crystals that convert the 511 kiloelectronvolt (keV) annihilation photons, forming a detector block.
In a clinical whole-body PET scanner, which consists of a large number of detector blocks arranged in a ring around the patient bore, each block conventionally shares timing, position and energy readout electronics to reduce the total number of readout channels for cost consideration. The coincidence resolving time (CRT) of each detector block is an important factor affecting the image quality of the reconstructed distribution of the positron-emitting radioisotope in patients.
Compared to conventional vacuum photomultiplier tube (PMT), SiPM has the advantages of compact size and allowing mass production with reduced variability and lower cost per unit of photosensitive area. Other superior characteristics are related to operation and performance, such as: higher photon detection efficiency (PDE), lower bias voltage, better timing resolution, and insensitivity to magnetic fields. However, SiPM has the disadvantages of higher dark count rate, slower fall time of output pulse relative to PMT, and signal-correlated spurious effects such as cross-talk and after-pulsing. These effects are cumulative with the number of SiPM devices connected into a PET detector block, and result in significant timing resolution degradation as the block size is increased.